The people of Bardo from season 7 of The 100 are set up as the antagonists, but with an air of mystery to them that ties back to the first nuclear apocalypse. The show has come a long way from that time, leaving Earth behind with a 125-year time jump between seasons 5 and 6, but its final season is tying back to the first in an unexpected way.
With the existence of the Anomaly, the main characters of The 100 have access to more planets than just Sanctum. The most important of those is Bardo, dominated by a religious group called “the Disciples”. While the original assumption is that the Disciples were descendants of an Eligius III crew, the show is instead pointing to a different origin for the group.
In season 7 episode 6 of The 100, “Nakara”, the connection between the Disciples and Earth is made stronger, suggesting that the Disciples are actually descendants of the Second Dawn who made it to Bardo using an Anomaly Stone on Earth. This would give them a very different origin, connecting them to Earth before the first nuclear apocalypse.
The Second Dawn was a doomsday cult introduced in The 100 season 4. When Praimfaya (deadly waves of radiation) was sweeping the planet, finding a bunker used by the Second Dawn was seen as one of the main opportunities they had to survive. Underneath the largest grounder city, Polis, was the main bunker that Second Dawn used, where most of the characters lived in the time jump between seasons 4 and 5.
In “Nakara”, Miller and Niylah recognize a Second Dawn symbol on the planet Nakara. In the previous episode, “Welcome to Bardo”, Anders says that the Shepherd saved them from the fire. These together suggest that members of the Second Dawn found an Anomaly Stone on Earth and then got to Bardo. The disciples even resemble the Second Dawn, having 12 “levels” of followers; only the level 12s went into the bunker with Cadogan, while Orlando proclaims Gabriel, Hope, and Echo to be “level 12” after their training on Skyring.
While this leaves a lot unanswered, like who the natives of Bardo are and what happened to them, it does give a solid backstory to where the Disciples on Bardo came from. It would also fit in with the backdoor pilot for The 100’s prequel spinoff, which tells the story of Earth right after the first nuclear apocalypse, by tying it in with the people of Bardo in the present day of the series.